1. Field of the Invention
The present invention relates to implantable hearing systems for assisting hearing in hearing-impaired persons and in particular to middle ear-implanted acoustic microphone systems with acoustic feedback prevention.
2. Description of Related Art
Some implantable hearing assistance systems use a microphone located in or near the ear to convert acoustic sound energy into an electrical signal. The electric signal is amplified, modulated and then communicated by a transducer to directly stimulate the cochlea to assist hearing. Alternatively, the amplified signal is communicated to a transducer for conversion to mechanical acoustic energy for vibratory application to a structure of the middle ear or the cochlea. The microphone can be located externally, subdermally adjacent the ear, or within the external auditory canal. The transducer is commonly connected to a portion of the middle ear, known as the ossicular chain, which includes the malleus, incus and stapes. Vibrations are emitted from the transducer into and through the ossicular chain to the cochlea of the inner ear.
The ossicular chain facilitates forward transmission of mechanical sound vibrations from the tympanic membrane of the external auditory canal to the inner ear. However, the ossicular chain also permits reverse transmission of mechanical sound energy to be transmitted from the transducer of the implantable hearing assistance system, back through the ossicular chain to the tympanic membrane, and into the external auditory canal. This retrograde sound transmission passes out of the external auditory canal and is acoustically fed back to the microphone of the system.
This acoustic feedback limits the maximum gain which the hearing assistance system can apply to the signal received by the microphone. In particular, the feedback created by reverse bone conduction through the ossicular chain has an inverse relationship with usable gain. For example, if one percent of the acoustic vibratory signal emitted by the transducer to the stapes, or other part of the ossicular chain, is fed back through the ossicular chain and into the external auditory canal to the microphone, the gain for the hearing assistance system is limited to roughly 100 or 40 dB. Due to the nature of the hearing losses and the acoustic limitations of these systems, a much higher gain is ideal. Accordingly, reduction or elimination of this feedback is desirable.
Moreover, these hearing assistance systems, which transmit acoustic sound energy onto an ossicular chain with a transducer, are inefficient and consume power rapidly. Inefficiency results from the mechanical force that must be exerted by the transducer against the ossicular chain and/or the tympanic membrane (in the case of microphone transducers located in the external auditory canal). This inefficiency causes rapid power consumption, requiring frequent battery changes. Battery changes are, at least, inconvenient for an externally located battery, and at worst, costly and surgically-related for a battery implanted in the middle ear or subdermally.
The importance of restoring hearing to hearing-impaired persons demands more optimal solutions in hearing assistance systems. Ideally, an improved hearing assistance system both minimizes power consumption as well as maximizes gain to produce a better acoustic signal for reception into the cochlea and the inner ear.